SiS 5591 chipset was sort of a forgotten Socket 7 chipset of the 100 MHz Socket 7 era chipset.
I believe the initial version didn't run at 100 MHz for the Socket 7 interface (only at 83 MHz), and I also remember that pretty much only GIGABYTE sold the version the fixed the 100 MHz bus clock support as GA-5SG100.
Perhaps PCChips (ECS's other mainboard brand) also sold mainboards SiS 5591 chipset with a different name, but I always avoided PCChips mainboard during that period (i.e., high failure rate).
How stable was SiS 5591 chipset's AGP?